Define proper binary operation APIs for columns

[vyasr]

This PR changes the way that binary operations are performed between columns. Instead of directly invoking the _binaryop method Frame binary operations now invoke operators directly using the operator module. Each Column subclass now only defines operations that are well-defined, relying on Python to handle raising TypeErrors for all others. Binary operations return NotImplemented instead of raising a TypeError except in specific cases where a meaningful error should be raised, allowing us to take advantage of reflected operations to prevent duplicate logic on how to handle binary operations between distinct types. Finally, various edge cases that were previously handled by Frames are now handled in Column so that different dtype columns are the sole source of truth on what operands are supported. These changes move us towards fully functional Column classes that do not rely on preprocessed inputs coming from the Frame layer.

This PR has a large changeset, but a large chunk of the changes lines are simply because some changes to the pipeline result in operations having their dunder names instead of having the dunders stripped, e.g. __add__ instead of add.

[bdice]

Comments attached. Overall this looks like a good overhaul of the binary op dispatching logic.

[bdice]

I'd suggest a different function name here. It doesn't include NULL_EQUALS and is pretty long / confusing.

Suggested change

	out_dtype = self._binary_op_lt_gt_le_ge_eq_ne(other)
	out_dtype = self._binary_op_comparison(other)

[vyasr]

I'm going to push this to a future PR as well. I'm hoping to improve the per-column binary operation logic in a number of places, but I'm trying to restrict this PR to just sorting out the public API so that we can decouple future changes to the Frame layer from changes to the Column layer.

[bdice]

All looks fine. Thanks for your replies to comments. I'll let you click "Resolve" on the remaining conversations, or leave them open if it helps you track ideas for follow-up work.

[codecov]

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[vyasr]

rerun tests

[vyasr]

@gpucibot merge